1. Technical Field of the Invention
The present invention relates to a degassing centrifugal apparatus such as a pump and to a process for degassing and pumping a liquid, especially backwater in the production of paper or board in a paper machine.
2. Prior Art
Backwater drained through a forming fabric in a papermaking process, normally, contains a large amount of entrained air. Since the short circulation of a paper machine requires a particularly constant flow, disturbing air is normally removed by conducting the drained backwater, by means of special pipe or channel systems, from the dewatering elements to an open backwater tank. The deaerated water is then pumped back to the fiber process of the short circulation preceding sheet forming. Other liquids which require air free pumping are, among others, black liquor, deinking sludge and stock, coating colour, etc.
Pumps, which are able to separate gas from a fluid to be pumped are well known as such, but the objective of such pumps is normally just to remove a sufficient portion of the gas to enable regular pumping. The known pumps are not capable of removing enough gas for achieving the degree of freedom of air, which is mostly required for wing the fluid directly for example in a papermaking process without ether deaeration.
Examples of prior art pumps capable of uniformly pumping fluids which contain gas are disclosed in Patents such as U.S. Pat. No. 4,410,337 and U.S. Pat. No. 5,039,320. Said pumps are so called MC-pumps developed for pumping high consistency (about 5% to 20%) pulp which must be fluidized in the suction channel of the pump, whereby air is separated through shear and centrifugal forces. The separated air concentrates in the center of the pump and is discharged by various means. Due to the small separation volume and high viscosity of the fluids to be pumped the separation of liquid and gas is not complete in the prior art pumps mentioned above. Consequently, separation of solid material and liquid from the discharged air is further required as taught for instance in Patent Applications EP 337394 and EP 298442.
The International Patent Application WO 92/03613 discloses a stock feeding arangement and process wherein a fiber suspension is pumped by means of xe2x80x9cmodifiedxe2x80x9d versions of the MC-pumps mentioned above. However, the specification does not explain how these pumps are to be modified.
Other means for separating gas from fluids, or for pumping fluids containing or developing vapor are disclosed in Patents such as U.S. Pat. Nos. 3,203,354, 3,323,465, 3,856,483, 4,201,555, 3,973,930, 4,516,987, 4,600,413, 4,675,033, 4,908,048 and WO 93/23135.
The same applicant""s U.S. Pat. No. 5,861,052, the disclosure of which is included herein by reference, discloses a gas separating pump capable of separating air and water from a mixture thereof. The pump has a fluid inlet at one end and a pumping liquid outlet at the opposite end. Between inlet and outlet there is a hollow elongated gas separating rotor and a generally central outlet for separated gas. At the outlet end of the pump the diameter of the rotor increases smoothly to form a larger diameter pumping zone. The inlet end of die pump is provided with a set of blades for distributing the incoming fluid to the rotor walls and the outlet of the pump Is provided with a blade wheel for pumping the degassed liquid. The apparatus is especially well suited for the gas-free re-cycling of backwater drained through a forming fabric in a papermaking process.
A variant of the above mentioned degassing pump is described in the same applicant""s earlier patent application WO 96/19276 wherein a threshold means is provided for ascertaining that the liquid flow at the outlet end is i a tranquil flow mode. The threshold means defines the position at which the transition from rapid tranquil flow will take place via a hydraulic jump.
The hydraulic jump is described, for instance by B. S. Massey; Mechanics of Fluids, 2nd Edition, Van Norsrtand Reinhold Co, Ltd. London 1971, pp 334 to 359, who teaches that a fluid flowing in an open channel cam flow in two distinct modes, either tranquil or rapid. The flow mode of the fluid is determined by the Froude number according to the equation.
Fr=u/(hxc3x97g)1/2
where Fr is the Froude number, u is the flow speed, h is the layer thickness and g is the force of gravitation. If Fr greater than 1 the flow is rapid and if Fr less than 1 the flow is tranquil. At a given energy level there is no intermediate flow speed, which means that if a rapid flow is slowed down, a hydraulic jump will take place at the transition between rapid and tranquil flow whereby part of the energy gets lost in turbulence. This phenomenon is known in the art of fluid flow in open channels, and it is utilized among others in water power stations,
Since pumps generally operate in a totally fluid filled state and have no open fluid surface there exists no hydraulic jump which is characteristic only for flow in open channels.
Thus, the phenomenon has not previously been utilized in connection with centrifugal separation of gases from liquids nor in the pumping of liquids.
If the liquid flow in the gas separation drum of a centrifugal gas separation device is rapid at any stage, it should preferably be transformed into a tranquil state before being re-moved from the device in order to avoid excessive turbulence and retaining of gas into the liquid at the outlet end. This is particularly important in cases, where the objective is to produce a liquid essentially free of gas, as is the case for instance in the air-free pumping of back-water of a paper machine.
Most prior art pumps operate in a filled state. Some pumping devices with an open surface are known but their axial flow is generally so slow that tranquil flow prevails, or the need for a gas-free state is secondary and a rapid flow at the outlet can be accepted.
In prior art open pumps with a high flow rate and a high demand for gas-free liquid, a hydraulic jump will obviously occur at some stage. The exact place of the hydraulic jump varies with the flow conditions and the shape of the air separation drum, and sometimes the hydraulic jump may be instable, causing instability in the function of the pump. The gas separation pump according to the above mentioned earlier parent application WO 96/19276 has an annular threshold m s for defining the position of the hydraulic jump.
An object of the present invention is to improve the function of known processes and apparatuses in order to provide stable conditions when separating a gas from a liquid at a high flow rate. The object of the invention is especially to control the hydraulic conditions in a gas separation device so as to reduce the energy losses. It is a special object of the invention to control the hydraulic conditions in a degassing pump and to reduce the energy losses such as those caused by the hydraulic jump.
The preferred embodiment of the present invention is based on the realization that the energy losses caused by the hydraulic jump may be reduced or eliminated by modifying the geometry of the inlet means of a degassing centrifugal pump. The unique features of the present invention are defined in the appended claims.
Thus, the present invention relates to a degassing centrifugal apparatus comprising a rotatable hollow rotor connected to a stationary fluid inlet at one end and a stationary liquid outlet at the opposite end, and having a gas exhaust in the center thereof, said rotor having at its inlet end a bladed wheel for rotating a fluid in said rotor. According to the invention said bladed wheel comprises a shovel wheel for accelerating the fluid flow in said rotor inlet end and causing said fluid to rotate at a peripherical velocity higher than the peripherical velocity of said rotor inlet end. The shovel wheel has a plurality of shovels extending inwards from said inner wall of said rotor and having an arcuate shape with a leading edge directed towards said stationary inlet and an outlet edge directed towards said inner wall, said outlet edge forming an angle with a line parallel to the centerline of said rotor.
In a preferred embodiment of the invention, the inlet end of said apparatus additionally comprises centrally located throttle means for directing an incoming fluid flow away from the center of the rotor and towards the inner wall of the rotor.
According to a special embodiment of the invention the rotor may comprise two separate rotating bodies rotatable at different speeds, the inlet shovel wheel being attached to the wall of the upstream one of said rotor bodies. The apparatus of the present invention is preferably used as a pump and the outlet end of the apparatus therefor preferably comprises a stationary spiralled peripheral outlet for pumping the degassed liquid.
The outlet end of the rotor preferably comprises a pumping wheel, a turbine wheel or it may also lack a wheel altogether at the outlet end. Thus, the pump according to the present invention may have an enlarged rotor outlet with an enlarged pumping zone as discussed in U.S. Pat. No. 5,861,052 although this is not necessary. In fact, the rotating pump may at the outlet end be configured as a substantially straight tube without enlargement and without any blade wheel at all. Such a pump will, in certain conditions perform its degassing and pumping action in a totally satisfactory manner.
When a fluid is accelerated by means of a shovel, the speed difference between the shovel and the fluid will remain constant, but the flow will change direction according to the shape of the shovel. In a rotating shovel wheel, the speed of the shovel is higher at the periphery than in the center, and the speed difference changes correspondingly.
By modifying the geometry of the shovels of the inlet shovel wheel of the pump, the direction of the flow differential can be directed more peripherically and less axially, as a result of which the rotating speed of the liquid will be higher than the rotating speed of the rotor at this position
When the fluid rotates faster than the rotor, the centrifugal force and, thus, the outlet pressure is increased. Also the dynamic energy component, which is transformed to pressure in the outlet pumping spiral, is increased.
In prior art centrifuges, the fluid has been brought to rotate at a rotational speed corresponding to that of the centrifuge. Now we have found, that this causes the fluid fed into the centrifuge to flow with an axial velocity corresponding to the difference between inlet flow speed and the peripherical speed of the centrifuge. This leads to a very rapid flow, which has to be reduced to a tranquil one over a hydraulic jump.
A part of the energy of the axial flow is lost in the hydraulic jump, causing a turbulence. The bigger the axial energy and the lower the centripetal force, the higher will be the hydraulic jump, the bigger the energy loss and the more violent the turbulence. In centrifuges for degassing, like the deaerating pump of U.S. Pat. No. 5,861,052 it has been found, that when the inlet speed is high and the capacity big, the hydraulic jump may re-introduce air into the fluid to be deaerated.
By turning the flow in the direction of the rotation of the centrifuge, the axial flow speed may be reduced, and at the same time, the peripherical flow speed increased. In a preferred embodiment the shovels are designed so as to provide a very high rotational speed component and an axial speed component which is low enough to provide an initial tranquil flow on the rotor wall. In such a case there will be no hydraulic jump and the energy losses are minimized.
The present invention also relates to a process for degassing a fluid by centrifuging, feeding a fluid containing a mixture of liquid and gas into a rotating inlet end of a rotating hollow rotor, accelerating the fluid flow in said rotor inlet end to cause said fluid to rotate at a peripherical velocity higher than the peripherical velocity of said rotor inlet end, bringing said fluid to flow axially along the inner wall of said rotor towards a liquid outlet at the opposite end of said rotor while causing said gas to separate from said liquid by centrifugal force, discharging the resulting degassed liquid at said rotor outlet end, and discharging the gas through a gas exhaust. The liquid is preferably discharged peripherally and the gas centrally.
By using the pump according to the present invention it is possible to provide an essentially complete separation of the gas which is included in the liquid in gaseous (non-dissolved) form from said fluid mixture.
The present invention also relates to improvements in a process for producing paper or board in a paper machine include the steps of providing a papermaking stock of pulp; diluting said stock in one or more stages with backwater drained through a forming wire of said paper machine; feeding said stock through a head box of said paper machine onto said forming wire; forming a web on said forming wire while allowing water from said stock to drain through said wire; feeding said web through a press section and a drying section of said paper machine to provide paper or board. The improvement comprises pumping at least a portion of said backwater and/or diluted stock with at least one degassing centrifugal pump in accordance with the present invention.
In a preferred embodiment of the invention the backwater is substantially completely degassed in said pump. The pumping may be performed with one, or preferably with several degassing centrifugal pumps.
A preferred embodiment of the process is provided by connecting the gas discharge to a vacuum source which may be used either to improve the effect of gas separation in the pump, or as a means of suction in a process upstream of the pump, or as means to provide a pressure drop in the inlet sufficient for distributing the fluid over the periphery of the same.